1. Field of the Invention
The present invention relates to a transparent recording medium providing recording with absorption in the visible range directly by heat or light.
2. Description of the Background Art
Conventionally, the following recording media are known for obtaining recording with absorption in the visible range directly by heat or light.
For a heat-sensitive recording medium used with a facsimile machine or the like, a colorless or light-colored electron donor dye (leuco dye) and a developer are dispersed and mixed in an aqueous solution of a water-soluble binder, and pulverized into minute particles to obtain a painting liquid. The painting liquid is applied to a nontransparent support member such as paper.
Japanese Patent Laid-open (ko-kai) Nos. 294088/1989 and 45087/1988 disclose a transparent heat sensitive recording medium prepared by dissolving a dye and a developer in an organic solvent and by applying to a transparent medium, wherein a nitrogen- or sulfur-containing onium compound is used as both a developer and thermal activation agent.
These are materials for recording the image by the direct heating method in which a thermal head is brought into contact with a recording medium and the medium is directly heated.
On the other hand, media and methods for recording an image by irradiating near-infrared light without direct heating are disclosed in Japanese Patent Laid-open (ko-kai) Nos. 94494/1983, 2880/1984, and 42994/1983. Japanese Patent Laid-open (ko-kai) No. 94494/1983 discloses an optical recording medium prepared by applying a coating liquid comprising a near-infrared absorbing agent dispersed in a water soluble binder, in addition to a leuco dye and a developer, to a support member. Japanese Patent Laid-open (ko-kai) Nos. 2880/1984 and 42994/1983 disclose optical recording media prepared by individually laminating a dye, a developer, and a near-infrared absorbing agent on a substrate by vacuum evaporation. Also, Japanese Patent Laid-open (ko-kai) No. 209594/1983 discloses a heat-sensitive recording medium wherein background fogging is prevented by the provision of an isolation layer prepared by coating a near-infrared absorbing agent in the interface of a coating layer, prepared by applying a solution of a color developer, e.g., a leuco dye, in an organic solvent to a substrate, and a layer of a developer solution provided on the coating layer.
However, the recording layer of a normal heat-sensitive recording medium comprising an electron donor colorless dye and a developer is opaque, because such a layer uses a dispersed coating liquid prepared by adding a developer and a dye reduced to minute particles in an aqueous medium, together with an auxiliary or filler. Such a recording medium therefore cannot function as a transparent recording medium. If a dye and a developer are simply dissolved in an organic solvent, the dye and the developer react with each other and develop a color so that such a composition cannot be used as a recording medium coating liquid.
On the other hand, in a composition in which an onium compound, as a developer, and a dye are dissolved together in an organic solvent, the onium salt does not react with the dye and no color is developed under normal conditions. Therefore, the composition can be used as a transparent heat-sensitive recording medium coating liquid. However, the decomposition reaction of the onium salt by the thermal energy is rather slow; no quick and adequate decomposition reaction is initiated by a thermal lead or by thermal energy produced by a light-heat conversion process by a semiconductor laser. Thus, it is essentially impossible to obtain a practical recording material with good sensitivity from such a composition. Furthermore, safety of the material is a problem, since an onium salt must be bonded to antimony hexafluoride, phosphorus hexafluoride, arsenic hexafluoride, or the like, as a counter anion.
The composition prepared by dispersing a dye, a developer and a near-infrared absorbing agent together with a binder is nontransparent. Therefore, such a composition cannot be used as a recording layer of a transparent recording medium. The method respectively laminating a dye, a developer, and a near-infrared absorbing agent by vacuum evaporation onto a substrate is unsuitable as a method for obtaining a normal transparent recording member from the aspect of transparency of the recording layers and cost.
High cost and incapability of mass production are problems in the transparent heat-sensitive recording medium in which an isolation layer of a near-infrared absorbing agent is provided between a dye layer and a developer layer, on account of the requirement of providing at least three layers. Another problem in this type of recording medium is contamination of components in one layer into other layers when coating is carried out, because colores are developed between the layers by the contamination.
Accordingly, the object of the present invention is to provide, with due consideration to the drawbacks of such conventional transparent recording media, a transparent recording medium on which a precise recording with an absorption in the visible range can be obtained directly by means of light or heat, using a coating composition comprising all necessary materials completely dissolved in a single solution.